The present invention relates to image sensors and, more particularly, to image sensors with current sources for biasing pixel array transistors.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an image sensor has an array of image sensor pixels. Each image sensor pixel has a photosensitive element for capturing light and converting the light into stored charge. Each image sensor pixel also has readout circuitry. The readout circuitry of each image sensor pixel typically includes a source-follower transistor that supplies a pixel output voltage to one of the output columns of the array. Proper biasing of the source-followers can be performed using current sources. The signals from the output columns can be amplified using amplifier circuitry.
The current sources that are used for biasing the source-follower transistors are typically each formed from a metal-oxide-semiconductor (MOS) transistor characterized by a transconductance parameter gm and a saturation voltage parameter vdsat. These parameters are related. A transistor with a large gm value will generally have a low value of vdsat and a transistor with a small gm value will generally have a large value of vdsat.
Care must be taken when designing the current source transistors. In bright image conditions, the output voltage swing from the image pixels will be large. If a current source transistor has a vdsat value that is too large, the current source will be saturated and will not be able to accommodate the large output voltage swing. In dark light conditions, on the other hand, large gm values (and therefore small vdsat values) can be associated with undesirable current source noise.
It would therefore be desirable to be able to provide current source circuitry and other image sensor circuitry that is able to operate effectively in a wide variety of lighting conditions.